The combustor of a gas turbine engine is exposed to local gas temperatures which commonly approach 3,500.degree. F. Rapid and wide ranging thermal excursions during heat up and cool down of the engine result in the cyclic exposure of combustor components to thermal shock and to high thermal stresses. At operating temperature, the combustor liner must support a steep thermal gradient across the liner from the hot inner surface to the cooler outer surface. Although the combustor does not experience a high mechanical load, the large thermal distortion of the components under operating conditions requires that the combustor exhibit elevated temperature load-carrying ability. In addition, the combustor is subjected to hot corrosive gases which chemically attack and mechanically erode the combustor wall.
The continually higher temperatures experienced in advanced gas turbine engines have carried combustor material requirements to the point at which even new and exotic metal alloys cannot effectively and economically provide the performance requirements and lifetimes required. The highest performance combustor liners are limited to a surface temperature of about 2,200.degree. F., so that the metal alloy combustor liners must be cooled by passing large quantities of cooling air over the inner and outer surfaces of the liners to ensure that the combustor wall temperature does not exceed the capabilities of the metal alloy. To operate at higher temperatures would require more cooling air to be diverted from the engine airflow, with a consequent degradation in engine performance, turbine durability, and increased engine emissions.
Ceramic materials are attractive materials for high temperature applications due to their characteristic high thermal stability. In the co-pending U.S. patent application Ser. No. 07/136,307, of common assignee herewith (currently under a U.S.P.T.O. Secrecy Order), ceramic tiles mounted to a fiber-reinforced substrate are used as panels to line the inside wall of the combustor. The ceramic tiles are embedded in the substrate support panel prior to firing the substrate, with the tiles and the substrate being in intimate contact with each other during the fabrication and firing processes. While this provides an improved combustor with significantly increased operating temperature capability, the contact between the tiles and the substrate provides a direct path for heat transfer from the tiles to the substrate.
What is needed is a combustor liner fabricated so as to minimize the direct contact between the tiles and the substrate so that the direct conduction of heat from the tiles to the substrate is reduced. This would permit the combustor to operate at higher temperatures without increasing the cooling air requirements, thus improving the performance of the engine.